In recent years, along with the glowing concern about environmental problems, a demand for reducing the amount of a carbon dioxide gas discharged from automobiles is increasing. For satisfying such a requirement, it is also demanded in terms of the tire performance to reduce the rolling resistance. As for the tire performance, the rolling resistance and the skid resistance on a road surface are competing performances, and trade-off balance therebetween is important. At the same time, improvement of wear resistance is also a big problem to be solved.
As regards the technique for improving the balance between the rolling resistance and the skid resistance on a wet road surface, a method by optimizing the tire structure has been also studied, but the method most commonly employed is to use a low heat-generating material as the rubber composition. Above all, a method of modifying the end of a conjugated diene-based polymer with a functional group is becoming popular as a most representative technique.
As regards the technique for introducing a functional group into the end of a conjugated diene-based polymer, a method described, for example, in WO01-23467, is used in general, where polymerization of 1,3-butadiene or copolymerization of 1,3-butadiene and styrene is performed using an organic lithium catalyst in a hydrocarbon solvent, and the active end of the polymer and a modifier reactable therewith are reacted, thereby introducing a functional group. Various modified conjugated diene-based polymers suitable for various applications have been proposed using such a technique. However, improvement of the wear resistance is not satisfactorily achieved.
On the other hand, an attempt has been made to improve various performances by increasing the molecular weight. However, the anionic polymerization using an organic lithium catalyst has a problem that trace impurities, particularly water, acetylenes, allenes and the like, in the raw material monomer react with the active lithium end of the polymer to inactivate the active end. Also, the active lithium end sometimes causes a reaction of decomposing by itself to produce lithium hydride and kill the end. In particular, such a side reaction becomes a serious trouble at the production of a modified conjugated diene-based polymer, because the absolute amount of the active end needs to be reduced for increasing the molecular weight. Thus, there is a problem that as the molecular weight is made higher, the residual ratio of active lithium end at the completion of polymerization is liable to decrease. Dehydration and purification of the monomer and solvent are important, but these are not sufficiently effective in the industrial-scale production and on the other hand, a tremendous cost is economically necessary at present for obtaining sufficiently high effects. In order to solve these impurity problems, JP-A-59-176311 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) has proposed to bring 1,3-butadiene and a hydrocarbon solvent into mixing contact with an organic lithium compound and then perform the polymerization. Also, JP-A-11-199712 has proposed coupling by various functional compounds for increasing the molecular weight. However, an effective countermeasure is not taken against the problem that the active lithium end causes a reaction of decomposing by itself to produce lithium hydride and kill the end. It is still difficult at present to achieve both a molecular weight increase and a high modification ratio at the same time.
Patent Document 1: WO01-23467
Patent Document 2: JP-A-59-176311
Patent Document 3: JP-A-11-199712